This application is based on and incorporates herein by reference Japanese Patent Application No. 11-366445 filed Dec. 24, 1999.
The present invention relates to a stick-type ignition coil device that is fit in a plug hole of each cylinder of an engine.
A stick-type ignition coil device that is fit in a plug hole of each cylinder of an internal combustion engine is constructed as shown in FIG. 3. Specifically, the ignition coil device is generally constructed with cylindrical coil casing 11, a central core 18, a primary spool 19, a primary winding 20 wound about the primary spool 19, a secondary spool 21 and a secondary winding 22 wound about the secondary spool 21. These components are disposed concentrically. A connector housing 14 is formed with a positioning part 16 and assembled with the coil casing 11 in such a manner that the positioning part 16 holds the central core 18 in central position through a cushion 23. The coil casing 11, the connector housing 14 and the spools 19, 21 are made of resin. An insulating resin 24 is filled in spaces among those components for electrical insulation and fixing.
The ignition coil device repeats to generate heat in high voltage generating operation and radiate heat after the operation. As a result, the insulating resin 24, the positioning part 16 and the like repeat thermal expansion and contraction. As the positioning part 16 and the insulating resin 24 have different coefficients of thermal expansion, thermal stress exerts from the insulating resin 24 to the positioning part 16 due to temperature changes. The central core 18 and the cushion 23 displace in the downward direction in FIG. 3 relative to the positioning part 16 due to difference between the coefficients of thermal expansion, when the temperature rises in the ignition coil operation.
As the positioning part 16 is firmly fixed to the insulating resin 24, the positioning part 16 is restricted from displacing by the insulating resin 24 covering the upper surface of the positioning part 16 and the positioning part 16 does not deform. As a result, the insulating resin layer 24a is subjected to pull stress in the upward and downward directions thus causing cracks X in the insulating resin layer 24a between the positioning part 16 and the cushion 23 as shown in FIG. 4. If the cracks X progress in the longitudinal direction along the central core 18, the electrical insulation between the central core 18 and the secondary winding 22 which generates a high ignition voltage is lessened due to dielectric breakdown Y across the spool 21.
It is therefore an object of the present invention to provide a stick-type ignition coil device, which reduces generation of cracks and ensures electrical insulation.
According to the present invention, a stick-type ignition coil device for engines comprises a central core, primary and secondary windings, a positioning member disposed to define a position of the central core, a coil casing encasing these components therein, and an insulating resin filled in spaces in the coil casing. A thermal stress releasing member is disposed on the upper surface of the positioning member to release thermal stress which exerts between the positioning member and the insulating resin covering the positioning member.
Preferably, the thermal stress releasing member is made of a material which is less adhesive to the insulating resin and the positioning member. The thermal stress releasing member is larger than an end surface of the central core in size and provided right above the central core.